The present invention relates to a curvilinear bearing unit for providing a guide for rotational or curvilinear motion of a machine part around a stationary part.
Hitherto, various types of swivel-type bearing apparatus have been proposed and used for providing a smooth guide for a rotational machine part such as turn tables, as well as machine parts having arms which which make swivel motion within a limited angular range such as track cranes, welding robots, manipulators, medical instrument and so forth. One of these known bearing apparatus, referred to as "cross roller bearing", has V-shaped grooves spaced 90.degree. apart in a split-type inner race or outer race, and a plurality of rollers arranged in these grooves alternatingly in a crossing manner and retained by a spacer retainer, the spacer retainer separating adjacent rollers to prevent any skew of the rollers and to avoid friction between these rollers.
This cross roller bearing, however, suffers from the following disadvantages. Namely, since the rollers are arranged at a constant pitch around the inner race, this type of bearing can have only small resistance to a concentration of the load, although it has a large load carrying capacity when the load is uniformly distributed over the entire circumference of the inner race. In addition, since the outer race completely surrounds the inner race through the intermediary of the rollers, it is quite difficult to adjust the clearance or gap between the races and, hence, to suitably preload the bearing.
Another bearing which is usually referred to as "wire race ball bearing" is known as a bearing apparatus suitable for use in large-scale machines. In this bearing apparatus, four endless steel wires are disposed between an outer race and a split-type inner race. Balls are disposed to run along the tracks provided by the steel wires. Coil springs serving as spacer/retainers are disposed between respective adjacent balls. This type of bearing apparatus, however, requires a highly precise finishing of the junction between the ends of each wire. In addition, it is necessary to conduct a running-in before the apparatus is put into normal service, in order to obtain a good smoothness of the track surfaces provided by the endless wire steels.
Still another bearing apparatus referred to as "ball race type bearing" is known as a bearing apparatus suitable for large-scale machines. This type of bearing apparatus, however, is extremely difficult to fabricate particularly when the diameter of the outer race is large. Namely, since a large quench-strain is unavoidable, it is necessary to preserve a large grinding margin to eliminate the dimensional error due to the strain. A large grinding margin, however, causes a large distortion due to heat produced during the grinding. The grinding, therefore, has to be made with a small rate of feed of the grinding tool, taking an impractically long period of time. Furthermore, the quenching of the ring as the blank of the outer race has to be made by press quenching, in order to minimize the aforementioned quench strain.
It is to be pointed out also that, in these known bearing apparatus employing split type outer races, it is quite difficult to adjust the radial and axial gaps between the outer and inner races, particularly at the points where the opposing ends of two halves of the outer race are joined.